The high court has ruled that the Metropolitan police broke the law in the way they “kettled” protesters at the G20 demonstrations in 2009.

In a landmark judgment on Thursday, high court judges found for protesters who had claimed police treated them unfairly. It also criticised the use of force by officers.

In the case, the high court heard that officers used punches to the face, slaps and shields against demonstrators who police chiefs accept had nothing to do with violence. The judgment does not strike down the police tactic of kettling or mass detention, but it will be seen as a rebuff to the Met.

The judgment places limits on the use of kettling. It says: “The police may only take such preventive action as a last resort catering for situations about to descend into violence.”